flixborough shut down diaster

28 march, 1974 (shut down)
june explosion
fliborough, uk
28 fatalities, 89 injured
2yr Caprolactum (intermediate used in nylon manufacturing) plant is being placed back to service. the plant was plagued with leaks and issues.
nitrogen fertllisers ltd. -fission ltd+dutch state mines = Nypro ltd
60 acres site for ammonium nitrate originally, Nyrpo was touse this to make Caprolactum

a background.

plant was manufacturing- Caprolactum(20000 tonnes/yr) using phenol hydrogenation. company wanted to increase production using Cyclohexane oxidation. this required Nypro to make a new plant onsite.
so 6 reactors, out of which 1 needed to be shut down (reactor no5). due tocracks in the steel walls. 6 ft crack was found.
and when one shuts down a reactor, a elbow pipe with expanding bellows are used to provide a temporary bypass. this would make use of the present pipe and bellows but the bellows will be extended.
here, instead of following standard procedures, there was neglect for the engineering aspects of safety and the calculations were neglected and not checked by the british standards at that time.
Quote

British standards 3351- required the pipe to be tested using hydrostatic pressure of no less than ie safety valve operating pressure.
the pipe was tested using nitrogen!!
no leaks were detected when back to work (april) but no one paid attention to the bypass either.
installation was used continously.

concerns that werent addressed

management didnt know the reason for the intial crack and forgot to take that inti consideration.
no inspectiom of other reactors.
new bypass line, inlet and outlet were at significantly different heights as compared to orginal.

reactor design

6 in series, 35 cm below the previous. 45 capacity, 5m high, diameter of 3.5m.
walls- 13mm of steel with 3 mm on the insidemade of stainless steel.
711mm, 28 inch pipe and bellows
all 6 reactors were used. 25 cylcohexane was pressurised and heated to 8.8 bar, 150 ⁰C.
by 6th reactor, 6% of CH had reacted, remaining was recycled.
heat exchangers used steam. block valve was closed during startup. so safety valves and manual operation was needed.
off gas line, to equalise the pressure. venting of the gas to scrubber
this was a disappointment. lackluster. leaks.

Screenshot_20250109_230143_YouTube 2.jpg

the incident

leak detected- 29th may, shutdown, repair work on 30th and 31st.
after N2 pressure testing, found to be leak free.
1 hr later more leaks AND YET LEAK FREE.
reactors were started started again.
Pressure in reactor 1 rose again, yet another shutdown.
broken plant. tools were locked away.
7 am, 1st june. state of plant was unsatisfactory. new shift.
8 am- again nitrogen testing.
startup, heat ex- steam was fed.
12 pm- technician saw high pressure. (9.2 kg/cm2)
operator- opens the block valve. (pressure goes down)
reduction in steam, so temp went down.
at 16:51, bypass failed, releasing 60 tons of cyclohexane.
came in contact w reforming tower, producing hydrogen.
at 16:53, explosion upto 50 km. flames upto 100 m tall.
18 staff members. (weekend)
but on weekday (500 ppl would have died.)

Danger

1970s, 3 days of electricty/week. impellers were shut down.

video link
flixborough
more detailed